Simultaneous determination of the mechanical moduli and mass of thin layers using nonadditive quartz crystal acoustic impedance analysis

High frequency rheological properties of polymer films have been measured using a nonadditive quartz crystal resonator impedance approach. The method involves fast measurement of the quartz covered with the film in contact with solutions of different viscosities and the measurement of the equivalent impedance of the bare quartz resonator in contact with the same solutions. Although by using a standard quartz crystal acoustic impedance method it is not possible to determine more than two of the four film parameters (density, thickness, storage modulus G', and loss modulus G "), we demonstrate that the interaction between the film under study and a semi-infinite layer of liquid gives the additional information needed to get the mechanical moduli G' and G " simultaneously with the mass or thickness of the film. An alternative approach to measure G' and G ", which involves independent and very precise determination of the film thickness, is discussed. Experimental measurements of polystyrene using dioctyl phthalate as a plasticizer show excellent agreement with Martin's model for quartz resonators covered with viscoelastic nonpiezoelectric layers. The results demonstrate that quartz resonators can be used to measure high frequency rheological properties of films simply without the need of an independent measurement of thickness. (C) 1999 American Institute of Physics. [S0021-8979(99)07315-6].